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76     ON  THE  EFFECT  OF  THE  INTERNAL  FRICTION  OF  FLUIDS
PART II. COMPARISON   OF  THEORY  AND  EXPERIMENT.
SECTION I.
Discussion of the Experiments of Baity, Bessel, Coulomb, and
Dubuat*.
54. THE experiments discussed in this Section will be taken in the order which is most convenient for discussion, which happens to be almost exactly the reverse of the chronological order. I commence with the experiments of the late Mr Baily, which are described in the Philosophical Transactions for 1832, in a memoir entitled "On the Correction of a Pendulum for the Reduction to a Vacuum: together with Remarks on some anomalies observed in Pendulum experiments."
* [At the time when this paper was read, the relation between /UL and p cannot be said to have been known. It is true that it may be inferred (at least for air, and thence presumably for other gases) from certain of Graham's experiments on the transpiration of gases. These however had been but recently published, having appeared in the Philosophical Transactions for 1846; and it was not till many years afterwards, about 1859, that Maxwell first inferred from the kinetic theory of gases the law that bears his name, namely that the coefficient of viscosity ju. is independent of the density.
In the comparison of theory and experiment as regards the effect of the presence of air on the motion of pendulums, I relied mainly on the experiments of Baily, which were made by a direct method, while at the same time they were conducted with all the accuracy of modern physical research, and embraced a great variety of forms of pendulum, many of them such as to admit of comparison with theory.
These experiments were strictly differential, giving the difference between the time of vibration at atmospheric pressure and in rarefied air. Had the vacuum been absolutely perfect, the difference would have given at once the effect of air at the atmospheric pressure. Had it merely been very high, the effect of the residual air on the time of vibration would have been insensible, and the result as regards the time would still have been the same. It is true that the whole effect of the rarefied air would not thus disappear; as a result of Maxwell's law it would tend, as the exhaustion proceeded, to fall wholly on the arc of vibration, and to approach a finite limit; and this limit would not begin to break down till an exhaustion was reached comparable with the highest we have to deal with in radiometer vacua.
But in Baily's experiments no high exhaustions were aimed at; the air was merely pumped out till the pressure was reduced to about one inch of mercury, and